LEDs are used today as light sources for high performance lighting fixtures. There are known advantages of LEDs as a light source, such as high efficacy in terms of lumens per watt, small form factor and durability. Moreover, LED lifetimes may be significantly longer than incandescent light bulbs. For instance, it is reported that the average life time of a LED is approximately 50000 hours instead of 2000 hours of an incandescent light bulb. As a result, LEDs are becoming a preferred light source in difficult-to-replace lighting fixtures—such as street lights and traffic signal lights, and in fixtures that require higher reliability—such as automotive lights, for safety reasons.
Nonetheless, the spread of lifetimes is not negligible, and use LED strings, since a string is only as good as its worst LED. In many applications this can lead to issues or challenges in applications which, it is therefore desirable or necessary to detect when a single LED in the string fails, in order that the system can react in a suitable way. In applications where a string of different colour LEDs is used to provide a white light source—which may even be tuneable or dimmable—a failure of an LED would mean a shift in the total colour of the system; in other applications, in which each LED in a string of LEDs is separately visible—such as daytime running lights (DRL) in the automobile industry—there may be an aesthetic impact of the failure of a single LED.
Failure of an LED in a string of LEDs can generally occur in one of two ways: firstly, the LED may fail open-circuit in which case the entire string becomes open circuit, which is generally easy to detect; alternatively, the LED may fail in a closed-circuit mode. Such a failure may not be so easy to detect.
LED strings are commonly operated in either a constant current mode, or in a pulse width modulated (PWM) mode in which pulsed current is supplied through the string, with the current being switched off between pulses and there being a predetermined mark-space ratio of the pulses. The PWM may typically be operated to provide either a constant average current or a constant on-current. During the on-state, a short-circuit failure of an LED may be detected by means of a reduction in the voltage across the string, with the reduction corresponding to the forward voltage (in the on-state) of the LED just prior to its failure. However, the forward voltage across an operating LED is dependent not only on the current through it, but also on its junction temperature. So, for instance, it may not be possible to distinguish between a fully operational string of 20 LEDs each operating at a first junction temperature, and a string of 19 LEDs (together with one LED in which has failed in a short-circuit mode), at a second temperature 30° cooler than the first temperature. Further, if the failure occurs during a start-up phase, there may not be a well-determined or stable forward voltage across the string, just prior to the failure, so it may not be possible to detect a change in the voltage, and an LED failure may thus go undetected.